Composite structures are utilized in a wide variety of applications. In recent years, the variety of applications which utilize composite structure has increased as applications for composite technology have expanded from the aerospace industry to the civil, construction, marine and transportation industries. With respect to civil engineering applications, for example, composite structures have been utilized as prestressing tendons, as reinforcing bars and, most recently, as structural members of bridges that are either being newly constructed or are being rehabilitated.
In addition, an increasing number of composite structures are designed to be smart structures by including one or more electrical or optical devices. In order to permit communication with the embedded device, these smart structures also generally include one or more leads embedded within and extending outwardly from the composite structure. As used herein, leads include electrical leads or wires, optical fibers and other leads or cables having a relatively small diameter in comparison to the dimensions of the composite structure within which the lead is embedded.
For example, composite structures and, more particularly, those composite smart structures designed for civil engineering applications may include one or more sensors. In this regard, composite structures can include a variety of electrical and/or optical sensors for measuring a variety of physical phenomena. For example, the embedded sensors can measure the strain exerted upon a composite structure. In particular, the embedded sensors can be designed to measure the residual stress and strain in composite structures arising from the manufacturing process and the interaction between the reinforcing fibers and the surrounding matrix. However, the embedded sensors can also be designed to measure the stress and strain imparted to the composite structure following its installation. For example, composite structures having embedded sensors can be utilized to construct a bridge such that the embedded sensors measure the deformation of the bridge.
Since composite structures are increasingly being utilized in larger quantities, it is desirable to mass produce the composite structures so as to reduce the manufacturing costs and the time required for manufacture of the composite structures. As such, at least some composite structures are pultruded. For example, the rods, bars and other elongated members utilized in a variety of civil engineering applications, such as bridge construction, can be readily pultruded.
As known to those skilled in the art, a conventional pultrusion process wets the fibers with a resin prior to pulling the wet fibers through a heated die which cures the resin to form the resulting composite structure. Advantageously, the pultrusion of composite structures can be performed as a continuous process such that a number of composite structures are formed in a serial fashion, thereby further increasing the efficiency with which the composite structures are fabricated. As will be apparent, the plurality of interconnected composite structures must then be separated or cut into individual composite structures following the pultrusion process. While the separation of the composite structures following the pultrusion process is generally straight-forward, the separation of composite structures that include leads, such as electrical wires or optical fibers, is significantly more complicated since the leads must also be recovered following the separation of the composite structures.
Most attempts to recover the leads embedded within a pultruded composite structure by machining the end or edge portion of the composite structure have damaged either the composite structure, the leads or both. As a result, resin starvation techniques, such as described in more detail by U.S. Pat. No. 4,347,287 to Armand F. Lewis, et al., which issued Aug. 31, 1982, have been developed which alter the general pultrusion process by periodically removing the resin such that the resulting product includes a number of fully formed composite structures separated by sections of dry fibers. As will be apparent, once the resulting composite structures have been separated, the leads can be readily recovered from the sections of dry fiber.
Unfortunately, pultrusion processes that utilize resin starvation have difficulty forming composite structures of a predetermined length since resin that has been previously supplied will wet the fibers and create a solid part for a period of time following the removal of the resin. As such, pultrusion processes which utilize resin starvation can typically only form composite structures having lengths that are within +/+30 cm of a desired length. Since composite parts are generally manufactured to tolerances of, at most, +/-1 cm in length, pultrusion processes that utilize resin starvation are generally not acceptable.
In addition, pultrusion processes that utilize resin starvation typically create composite structures having end portions with a tapered shape since the fibers which form the outer portions of the composite structure dry prior to the fibers within the interior of the composite structure. Thus, the resulting composite structure has a rather unfinished appearance since the end portions are not square. Further, pultrusion processes which utilize resin starvation never operate in a steady state for very long since the resin is intermittently removed from the pultrusion process, thereby inhibiting proper quality control and potentially degrading the ultimate mechanical properties of the resulting composite structures.
Furthermore, the leads are not protected within the sections of dry fibers. Thus, the pulling and/or gripping mechanism which advances the composite structure along the pultrusion fabrication line can damage the leads. Since the leads oftentimes permit communication within embedded devices, such as sensors and/or actuators, any damage to the lead may also prevent effective communication with the embedded devices. Because the embedded devices are generally relatively expensive, the costs of the resulting pultruded composite structure will therefore be increased even though communication cannot be established with the embedded sensors and/or actuators.
While it is advantageous in many applications to embed devices and their associated leads within composite structures, it is sometimes difficult to recover the leads following the fabrication of composite structure. In this regard, even though pultrusion is an efficient process by which to fabricate a plurality of composite structures, the continuous nature of the pultrusion process makes it even more difficult to recover leads which are embedded within the resulting composite structures. Even though resin starvation can be utilized in order to create sections of dry fibers from which the leads can be readily recovered, pultrusion processes which utilize resin starvation still suffer from a number of deficiencies as described above.